1. Field of the Invention
The present invention relates to a nitride semiconductor laser element and more particularly relates to a nitride semiconductor laser element having a protective film with cavity end faces.
2. Background Information
With a nitride semiconductor laser element, there is a concern that the cavity end faces formed by cleavage or RIE (reactive ion etching) will have a lower band gap energy. If the band gap energy of the cavity end face is too low, the laser light that is supposed to be emitted will instead be absorbed by the cavity end face. The absorption generates heat in the cavity end face, leading to damage or deterioration of the semiconductor laser element that is typified by COD (catastrophic optical damage), which means that it is difficult to raise output or sustain continuous drive for an extended period.
Various ways of dealing with this problem have been proposed, such as a method in which an oxide film or nitride film that cuts light absorption at the cavity end face and functions as a window layer is formed as a protective film of the cavity end face, or a method in which the thickness of the protective film formed on the cavity end face is varied according to the optical density to be emitted.
With a nitride semiconductor laser element, however, even if a protective film is formed from a material capable of cutting light absorption at the cavity end face, when the protective film is formed as a monocrystal or polycrystal, a difference in its lattice constant from that of the nitride semiconductor of the cavity end face can cause cracks to develop in the protective film or in the nitride semiconductor layer, so that the film cannot function as an end face protective film, and the element characteristics suffer. Particularly when the protective film has good crystallinity, although the end face protective film will be able to reflect or transmit laser light favorably, the protective film itself will be susceptible to cracking, creating problems with reliability. Thus, in the past, it was difficult to produce a nitride semiconductor laser element having a protective film with good crystallinity.
On the other hand, when the protective film is an amorphous film, stress in the film can cause the protective film to peel, or heat generated when light is absorbed at the end face can drive up the temperature of the cavity end face, which decreases the band gap of the cavity end face, further increases the absorption of outputted light, accelerates end face deterioration, and inhibits the desired function from being performed.
Also, even if a protective film is formed covering the semiconductor layer surface to prevent the protective film from peeling, the film will still peel away from the ends of the semiconductor layer surface under prolonged drive or high output drive. Also, when a protective film is formed covering the semiconductor layer surface from the cavity end face, the corners of the laser element are subjected to stress, which causes lifting or peeling of the protective film at the corners.